Extreme-Scale Simulations of Giant Impacts with pkdgrav3

Giant impacts are a fundamental process in planetary system formation and evolution, shaping the architecture, internal structure, and dynamical state of planets across a wide range of masses and compositions. Their inherently nonlinear and multiphysics nature, combining self-gravity, hydrodynamics, thermodynamics, and material strength, makes them challenging to model with high physical fidelity. As these events occur under conditions that cannot be reproduced in laboratory experiments, large-scale numerical simulations constitute a key tool for investigating their outcomes. This thesis focuses on the development and application of a high-performance Smoothed Particle Hydrodynamics (SPH) framework within the N-body code pkdgrav3. The code is extended with a parallel hydrodynamics solver, a novel neighbour-finding scheme tightly integrated into the hierarchical gravity tree, and, in later stages, a material strength model with pressure-dependent yielding. Together, these developments yield a scalable, GPU-accelerated framework that consistently couples self-gravity, hydrodynamics, and solid-body physics within a single simulation, enabling calculations with up to billions of particles, thereby achieving high physical realism at unprecedented resolution. During the development of this framework, complementary scientific investigations were carried out using the established SPH code Gasoline, extended with appropriate equations of state. First, we investigated mantle-stripping collisions associated with the formation of Mercury-like planets. These results indicate more restrictive conditions than previously assumed for producing highly iron-enriched remnants. In a separate line of work, we explored Moon-forming impact scenarios through a systematic survey of parameter space. The outcomes are highly sensitive to impact angle, velocity, mass ratio, and pre-impact rotation, and reveal previously unrecognized regions that simultaneously satisfy multiple observational constraints of the Earth-Moon system. With the new SPH implementation in pkdgrav3 in place, a series of simulations was performed to investigate impact regimes that benefit from increased resolution and physical fidelity. Collisions between gas clumps in protoplanetary environments exhibit a wide spectrum of outcomes, ranging from mergers and erosive interactions to collapse triggered by dynamical compression. Simulations of giant impacts on Venus are used to assess whether its present-day slow retrograde rotation and lack of a substantial satellite are compatible with such an impact, based on post-impact spin and debris disk formation. Finally, ultra-high-resolution simulations of impacts onto proto-Jupiter are conducted to test hypotheses for the origin of its dilute core, demonstrating that heavy elements fail to mix efficiently with the envelope and instead re-settle rapidly, preventing long-lived core disruption. Overall, this thesis demonstrates that the newly developed SPH framework in pkdgrav3 provides a powerful, flexible, and robust tool for studying giant impacts across planetary regimes. By enabling the consistent treatment of hydrodynamics, self-gravity, and material strength at high resolution and excellent scalability, it opens the door to systematic exploration of collision processes and their role in shaping planetary systems.